1. Field of the Invention
The present invention relates to a compact zone lens system suitable for use in lens-shutter cameras and video cameras. More particularly, the present invention relates to a compact zoom lens system which provides good correction of various aberrations and minimizes the length and the diameter of the lens barrel thereof in a rest position wherein the lens barrel is retracted to the maximum into the camera body.
2. Related Art
Recently, as lens-shutter cameras, video cameras and the like tend to be designed as compact as possible, it has been sought to provide the camera with a compact zoom lens system. Especially in lens-shutter cameras, it has been sought to make zoom cameras as small as conventional fixed-focus cameras. For this purpose, it is desirable to shorten the length of the lens barrel of the zoom lens system in the rest position and to minimize the diameter of the lens barrel.
It is necessary, for minimizing the diameter of the lens barrel, to construct the zoom lens system in such a manner as to simplify the construction of the lens barrel, not to mention the reduction of the diameter of the lenses. To achieve a simple construction of the lens barrel, various types of such zoom lens systems are known, for example, from Japanese Laid-Open Patent Appln. 56-128911, that consist of a forward lens group of a positive power and a rearward lens group of a negative power arranged in this order from the object side.
However, because the telephoto ratio in the telephoto mode is above 1 in these prior art systems, they are unsuitable for shortening the length of the lens barrel in the rest position. It is to be noted herein that the telephoto ratio means the value of the overall length of the lens system including the back focal distance thereof divided by the focal length.
In a zoom lens system composed of a forward lens group of a positive power and a rearward lens group of a negative power, the entire length of the system including the back focal distance can be shorter the greater is the refracting power of the forward lens group. However, if the refracting power of the forward lens group is simply increased, there would result an extremely short back focal distance in the wide-angle mode, or an unnecessarily strong refracting power of the rearward lens group. In that case, curvature of the field worsens, and the fluctuation of various aberrations upon zooming becomes greater.
Japanese Laid-Open Patent Appln. 63-161422 and 2-284109 also disclose a compact zoom lens system composed of a forward lens group of a positive power and a rearward lens group of a negative power. However, because the latter lens group has a large diameter, the zoom lens systems of these prior devices are unsuitable for minimizing the diameter of the lens barrel. Moreover, the former zoom lens not only has a small zoom ratio, less than a factor of 2, but also has a high telephoto ratio in the telephoto mode. The latter zoom lens system has a telephoto ratio in the telephoto mode that is even greater than that of the former zoom lens, because the latter zoom lens system is a wide-angle zoom lens.
It is also widely known to use aspherical lenses for shortening the overall length of the lens system. However, if the aspherical lens is mode of a plastic resin, it is impossible to completely eliminate the shift of focus due to variations of temperature and humidity. On the other hand, an aspherical lens made of glass is expensive.